A few small enhancements

Hey! In celebration of finally building lnotify, I thought I'd offer a few small enhancements :)

1. Added support for Lua 5.2 (LUA_INC=lua5.2; make)
2. Made notification 'body' optional
3. New 'set_appname' function. takes one or two args to set app name for a single note, or all notes.
4. Allow string args for set_urgency ('low', 'normal', 'critical')